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Comparing linear free energy relationships for organic chemicals in soils: effects of soil and solute properties.

ABSTRACT: Sorption isotherm data were determined for a set of 8 aromatic organic compounds with varying physical chemical properties in three soils with organic matter of differing quantity and composition. The primary goals of this study were to test single and multiparameter linear free energy relationships on their ability to predict the observed sorption behavior on different types of natural sorbents across a range of solutes and concentrations and to relate the accuracy of the predictions to sorbent and solute structural descriptors. Organic carbon normalized sorption coefficients (K(OC)) predicted using both single and multiparameter LFERs were in good agreement with experimental data obtained at the highest tested aqueous concentrations (average deviation less than 0.1 log units over all solutes and sorbents), but deviations were more substantial (0.59-0.65 log units) at the lowest tested concentrations. For chlorinated benzenes there was a significant correlation between experiment-prediction discrepancies and the aromatic content of the soil organic matter measured by (13)C NMR, and the magnitude of the effect was similar to that observed previously for dissolved organic matter.

SUBMITTER: Faria IR

PROVIDER: S-EPMC2941804 | biostudies-literature | 2010 Sep

REPOSITORIES: biostudies-literature

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Comparing linear free energy relationships for organic chemicals in soils: effects of soil and solute properties.

Faria Isabel R IR Young Thomas M TM

Environmental science & technology 20100901 18

Sorption isotherm data were determined for a set of 8 aromatic organic compounds with varying physical chemical properties in three soils with organic matter of differing quantity and composition. The primary goals of this study were to test single and multiparameter linear free energy relationships on their ability to predict the observed sorption behavior on different types of natural sorbents across a range of solutes and concentrations and to relate the accuracy of the predictions to sorbent ...[more]

PMID: 20726509

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Project description:Linear free energy relationships (LFERs) for substituent effects on reactions that proceed through similar transition states provide insight into transition state structures. A classical approach to the analysis of LFERs showed that differences in the slopes of Brønsted correlations for addition of substituted alkyl alcohols to ring-substituted 1-phenylethyl carbocations and to the β-galactopyranosyl carbocation intermediate of reactions catalyzed by β-galactosidase provide evidence that the enzyme catalyst modifies the curvature of the energy surface at the saddle point for the transition state for nucleophile addition. We have worked to generalize the use of LFERs in the determination of enzyme mechanisms. The defining property of enzyme catalysts is their specificity for binding the transition state with a much higher affinity than the substrate. Triosephosphate isomerase (TIM), orotidine 5'-monophosphate decarboxylase (OMPDC), and glycerol 3-phosphate dehydrogenase (GPDH) show effective catalysis of reactions of phosphorylated substrates and strong phosphite dianion activation of reactions of phosphodianion truncated substrates, with rate constants kcat/Km (M-1 s-1) and kcat/KdKHPi (M-2 s-1), respectively. Good linear logarithmic correlations, with a slope of 1.1, between these kinetic parameters determined for reactions catalyzed by five or more variant forms of each catalyst are observed, where the protein substitutions are mainly at side chains which function to stabilize the cage complex between the enzyme and substrate. This shows that the enzyme-catalyzed reactions of a whole substrate and substrate pieces proceed through transition states of similar structures. It provides support for the proposal that the dianion binding energy of whole phosphodianion substrates and of phosphite dianion is used to drive the conversion of these protein catalysts from flexible and entropically rich ground states to stiff and catalytically active Michaelis complexes that show the same activity toward catalysis of the reactions of whole and phosphodianion truncated substrates. There is a good linear correlation, with a slope of 0.73, between values of the dissociation constants log Ki for release of the transition state analog phosphoglycolate (PGA) trianion and log kcat/Km for isomerization of GAP for wild-type and variants of TIM. This correlation shows that the substituted amino acid side chains act to stabilize the complex between TIM and the PGA trianion and that ca. 70% of this stabilization is observed at the transition state for substrate deprotonation. The correlation provides evidence that these side chains function to enhance the basicity of the E165 side chain of TIM, which deprotonates the bound carbon acid substrate. There is a good linear correlation, with a slope of 0.74, between the values of ΔG‡ and ΔG° determined by electron valence bond (EVB) calculations to model deprotonation of dihydroxyacetone phosphate (DHAP) in water and when bound to wild-type and variant forms of TIM to form the enediolate reaction intermediate. This correlation provides evidence that the stabilizing interactions of the transition state for TIM-catalyzed deprotonation of DHAP are optimized by placement of amino acid side chains in positions that provide for the maximum stabilization of the charged reaction intermediate, relative to the neutral substrate.

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Comparing linear free energy relationships for organic chemicals in soils: effects of soil and solute properties.

Publications

Comparing linear free energy relationships for organic chemicals in soils: effects of soil and solute properties.

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